Abstract

Norbormide (NRB) is a selective vasoconstrictor agent of the rat small vessels. The mechanisms underlying the selective vasoconstrictor effect of NRB are unknown. To investigate whether phospholipase C (PLC) signaling pathway plays a role in NRB-induced vasoconstriction, we performed experiments in NRB-contracted tissues, namely, rat caudal arteries (RCA) and smooth muscle cells derived from rat mesenteric arteries (MVSMCs). An NRB-insensitive vessel, namely rat aorta (RA), served as a control tissue. In RCA and RA we measured either isometric tension or formation of inositol phosphates (IPs), the latter taken as an index of PLC activation. In MVSMCs, we measured intracellular free calcium concentration ([Ca²⁺]_cyt). In the presence of external Ca²⁺, NRB (2–50 μM) stimulated IPs formation in RCA but not in RA, and increased [Ca²⁺]_cyt in MVSMCs. In the absence of external Ca²⁺, NRB (50 μM) increased IPs formation in RCA but was unable to increase [Ca²⁺]_cyt in MVSMCs. In RCA, in the presence of external Ca²⁺, NRB-induced contraction was inhibited by calphostin C (0.2–1 μM), an inhibitor of protein kinase C (PKC), and by SK&F 96365 (30 μM), an inhibitor of the store-operated calcium channels, but was poorly affected by verapamil, an L-type calcium channel blocker. However, verapamil was much more effective when external Ca²⁺ was substituted by Sr²⁺. These results suggest that NRB elicits its tissue and species-selective vasoconstrictor effect by stimulating PLC-PKC pathway and increasing Ca²⁺ influx through both verapamil-sensitive and -insensitive calcium channels. Ca²⁺release from sarcoplasmic reticulum seems not involved in NRB vasoconstriction.